Return-Path: Received: from smtp1.linuxfoundation.org (smtp1.linux-foundation.org [172.17.192.35]) by mail.linuxfoundation.org (Postfix) with ESMTPS id C0D8D258 for ; Tue, 28 Jun 2016 03:36:41 +0000 (UTC) X-Greylist: domain auto-whitelisted by SQLgrey-1.7.6 Received: from ozlabs.org (ozlabs.org [103.22.144.67]) by smtp1.linuxfoundation.org (Postfix) with ESMTPS id 371FB255 for ; Tue, 28 Jun 2016 03:36:41 +0000 (UTC) Received: by ozlabs.org (Postfix, from userid 1011) id 3rds1305Xnz9snl; Tue, 28 Jun 2016 13:36:38 +1000 (AEST) From: Rusty Russell To: "Jonas Schnelli" User-Agent: Notmuch/0.21 (http://notmuchmail.org) Emacs/24.5.1 (x86_64-pc-linux-gnu) Date: Tue, 28 Jun 2016 12:01:34 +0930 Message-ID: <87h9cecad5.fsf@rustcorp.com.au> MIME-Version: 1.0 Content-Type: text/plain X-Spam-Status: No, score=-5.5 required=5.0 tests=BAYES_00,RCVD_IN_DNSWL_MED, RP_MATCHES_RCVD autolearn=ham version=3.3.1 X-Spam-Checker-Version: SpamAssassin 3.3.1 (2010-03-16) on smtp1.linux-foundation.org Cc: bitcoin-dev@lists.linuxfoundation.org Subject: [bitcoin-dev] BIP 151 use of HMAC_SHA512 X-BeenThere: bitcoin-dev@lists.linuxfoundation.org X-Mailman-Version: 2.1.12 Precedence: list List-Id: Bitcoin Protocol Discussion List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Tue, 28 Jun 2016 03:36:41 -0000 To quote: > HMAC_SHA512(key=ecdh_secret|cipher-type,msg="encryption key"). > > K_1 must be the left 32bytes of the HMAC_SHA512 hash. > K_2 must be the right 32bytes of the HMAC_SHA512 hash. This seems a weak reason to introduce SHA512 to the mix. Can we just make: K_1 = HMAC_SHA256(key=ecdh_secret|cipher-type,msg="header encryption key") K_2 = HMAC_SHA256(key=ecdh_secret|cipher-type,msg="body encryption key") Thanks, Rusty.